The present invention relates to chemically modified electrodes for use as biosensors, such as have been applied in clinical chemistry testing and in other monitoring and control environments, and is more particularly directed to problems relating to electrode fouling and interference from other species present in complex matrices such as blood, serum, or fermentation broths.
Biosensors are devices for sensing such substances as electrolytes, gases, proteins, enzymes, metabolites, antibodies, and antigens. They find wide application in such diverse fields as clinical chemistry testing, bioreactor monitoring and control, fermentation control, and medical research.
A biosensor of the sort of interest here generally includes a base electrode and a biochemically discriminating element disposed about the electrode. Typically the biochemically discriminating element is provided by a membrane which surrounds the electrode. The biosensor is placed in contact with the sample substrate under investigation, and the membrane isolates and transforms the desired analyte into an electrically active species or otherwise generates an electrical potential, which is sensed and monitored by the base electrode. Familiar examples include the use of a glass membrane, which selects and traps the desired electrolyte cation, such as Na.sup.+, in the silicate matrix of the glass, thereby producing a charge separation which is sensed by the electrode, and the use of a polyvinyl chloride matrix incorporating the antibiotic molecule valinomycin for selectively extracting K.sup.+ ions.
Other examples of biosensors are provided by so-called enzyme electrodes, in which the membrane is provided by a composite containing one or more immobilized enzymes. The enzymes catalyze a reaction of the sample substrate to consume or to generate a redox-active species in an amount related to the concentration of the target analyte in the sample substrate. The redox-active species may then be detected by measuring current flow at the electrode mediated by the redox-active species. The development of other membranes for selectively discriminating and sensing other analytes is currently an area of active research.
Paralleling the development of new membranes is the development of improved base electrodes. One such development is the use of reticulated vitreous carbon (IIRVCII) as a base electrode. See, for example, the work of Wieck, Heider, and Yacynych, reported in Anal. Chem. Acta, Vol. 158, pp. 137 et sea. (1984), in which an RVC electrode is incorporated into an immobilized enzyme detector for a flow injection determination of glucose. RVC presents a complex microenvironment to the substances to be detected by the base electrode. Attempts have been made to modify the microenvironment for improved electrode performance. See, for example, the work of Will and Iacovangelo, appearing in the Journal of the Electrochemical Society, Vol. 131, pp. 590 et secf. (1984), which discloses the electrodeposition of zinc metal onto RVC surfaces for improved performance.
Notwithstanding progress in the development of new sensing agents and electrodes, biosensors--old and new--fail to achieve their theoretical peak performance because of interference of species other than the desired analyte which can contribute to the potential difference or to the current sensed by the base electrode, thereby compromising the selectivity or measurement precision of the biosensor, and because of the fouling of the base electrode with repeated use.